Android中ACTION_CANCEL的触发机制与滑出子view的情况
目录
- ACTION_CANCEL的触发时机
- 1,父view拦截事件
- 2,ACTION_DOWN初始化操作
- 3,在子View处理事件的过程中被从父View中移除时
- 4,子View被设置了PFLAG_CANCEL_NEXT_UP_EVENT标记时
- 滑出子View区域会发生什么?
- 结论:
看完本文你将了解:
- ACTION_CANCEL的触发时机
- 滑出子View区域会发生什么?为什么不响应onClick()事件
首先看一下官方的解释:
/** * Constant for {@link #getActionMasked}: The current gesture has been aborted. * You will not receive any more points in it. You should treat this as * an up event, but not perform any action that you normally would. */ public static final int ACTION_CANCEL = 3;
说人话就是:当前的手势被中止了,你不会再收到任何事件了,你可以把它当做一个ACTION_UP事件,但是不要执行正常情况下的逻辑。
ACTION_CANCEL的触发时机
有四种情况会触发ACTION_CANCEL
:
- 在子View处理事件的过程中,父View对事件拦截
- ACTION_DOWN初始化操作
- 在子View处理事件的过程中被从父View中移除时
- 子View被设置了PFLAG_CANCEL_NEXT_UP_EVENT标记时
1,父view拦截事件
首先要了解ViewGroup什么情况下会拦截事件,Look the Fuck Resource Code:
/** * {@inheritDoc} */ @Override public boolean dispatchTouchEvent(MotionEvent ev) { ... boolean handled = false; if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; ... // Check for interception. final boolean intercepted; // 判断条件一 if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; // 判断条件二 if (!disallowIntercept) { intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } ... } ... }
有两个条件
- MotionEvent.ACTION_DOWN事件或者mFirstTouchTarget非空也就是有子view在处理事件
- 子view没有做拦截,也就是没有调用
ViewParent#requestDisallowInterceptTouchEvent(true)
如果满足上面的两个条件才会执行onInterceptTouchEvent(ev)
。
如果ViewGroup拦截了事件,则intercepted
变量为true,接着往下看:
@Override public boolean dispatchTouchEvent(MotionEvent ev) { boolean handled = false; if (onFilterTouchEventForSecurity(ev)) { ... // Check for interception. final boolean intercepted; if (actionMasked == MotionEvent.ACTION_DOWN || mFirstTouchTarget != null) { final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0; if (!disallowIntercept) { // 当mFirstTouchTarget != null,也就是子view处理了事件 // 此时如果父ViewGroup拦截了事件,intercepted==true intercepted = onInterceptTouchEvent(ev); ev.setAction(action); // restore action in case it was changed } else { intercepted = false; } } else { // There are no touch targets and this action is not an initial down // so this view group continues to intercept touches. intercepted = true; } ... // Dispatch to touch targets. if (mFirstTouchTarget == null) { ... } else { // Dispatch to touch targets, excluding the new touch target if we already // dispatched to it. Cancel touch targets if necessary. TouchTarget predecessor = null; TouchTarget target = mFirstTouchTarget; while (target != null) { final TouchTarget next = target.next; if (alreadyDispatchedToNewTouchTarget && target == newTouchTarget) { ... } else { // 判断一:此时cancelChild == true final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; // 判断二:给child发送cancel事件 if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; } ... } ... } } ... } ... return handled; }
以上判断一处cancelChild
为true,然后进入判断二中一看究竟:
private boolean dispatchTransformedTouchEvent(MotionEvent event, boolean cancel, View child, int desiredPointerIdBits) { final boolean handled; // Canceling motions is a special case. We don't need to perform any transformations // or filtering. The important part is the action, not the contents. final int oldAction = event.getAction(); if (cancel || oldAction == MotionEvent.ACTION_CANCEL) { // 将event设置成ACTION_CANCEL event.setAction(MotionEvent.ACTION_CANCEL); if (child == null) { ... } else { // 分发给child handled = child.dispatchTouchEvent(event); } event.setAction(oldAction); return handled; } ... }
当参数cancel为ture时会将event设置为MotionEvent.ACTION_CANCEL,然后分发给child。
2,ACTION_DOWN初始化操作
public boolean dispatchTouchEvent(MotionEvent ev) { boolean handled = false; if (onFilterTouchEventForSecurity(ev)) { final int action = ev.getAction(); final int actionMasked = action & MotionEvent.ACTION_MASK; // Handle an initial down. if (actionMasked == MotionEvent.ACTION_DOWN) { // Throw away all previous state when starting a new touch gesture. // The framework may have dropped the up or cancel event for the previous gesture // due to an app switch, ANR, or some other state change. // 取消并清除所有的Touch目标 cancelAndClearTouchTargets(ev); resetTouchState(); } ... } ... }
系统可能会由于App切换、ANR等原因丢失了up,cancel事件。
因此需要在ACTION_DOWN时丢弃掉所有前面的状态,具体代码如下:
private void cancelAndClearTouchTargets(MotionEvent event) { if (mFirstTouchTarget != null) { boolean syntheticEvent = false; if (event == null) { final long now = SystemClock.uptimeMillis(); event = MotionEvent.obtain(now, now, MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0); event.setSource(InputDevice.SOURCE_TOUCHSCREEN); syntheticEvent = true; } for (TouchTarget target = mFirstTouchTarget; target != null; target = target.next) { resetCancelNextUpFlag(target.child); // 分发事件同情况一 dispatchTransformedTouchEvent(event, true, target.child, target.pointerIdBits); } ... } }
PS:在dispatchDetachedFromWindow()
中也会调用cancelAndClearTouchTargets()
3,在子View处理事件的过程中被从父View中移除时
public void removeView(View view) { if (removeViewInternal(view)) { requestLayout(); invalidate(true); } } private boolean removeViewInternal(View view) { final int index = indexOfChild(view); if (index >= 0) { removeViewInternal(index, view); return true; } return false; } private void removeViewInternal(int index, View view) { ... cancelTouchTarget(view); ... } private void cancelTouchTarget(View view) { TouchTarget predecessor = null; TouchTarget target = mFirstTouchTarget; while (target != null) { final TouchTarget next = target.next; if (target.child == view) { ... // 创建ACTION_CANCEL事件 MotionEvent event = MotionEvent.obtain(now, now, MotionEvent.ACTION_CANCEL, 0.0f, 0.0f, 0); event.setSource(InputDevice.SOURCE_TOUCHSCREEN); 分发给目标view view.dispatchTouchEvent(event); event.recycle(); return; } predecessor = target; target = next; } }
4,子View被设置了PFLAG_CANCEL_NEXT_UP_EVENT标记时
在情况一种的两个判断处:
// 判断一:此时cancelChild == true final boolean cancelChild = resetCancelNextUpFlag(target.child) || intercepted; // 判断二:给child发送cancel事件 if (dispatchTransformedTouchEvent(ev, cancelChild, target.child, target.pointerIdBits)) { handled = true; }
当 resetCancelNextUpFlag(target.child)
为true时同样也会导致cancel,查看代码:
/** * Indicates whether the view is temporarily detached. * * @hide */ static final int PFLAG_CANCEL_NEXT_UP_EVENT = 0x04000000; private static boolean resetCancelNextUpFlag(View view) { if ((view.mPrivateFlags & PFLAG_CANCEL_NEXT_UP_EVENT) != 0) { view.mPrivateFlags &= ~PFLAG_CANCEL_NEXT_UP_EVENT; return true; } return false; }
根据注释大概意思是,该view暂时detached,detached是什么意思?就是和attached相反的那个,具体什么时候打了这个标记,我觉得没必要深究。
以上四种情况最重要的就是第一种,后面的只需了解即可。
滑出子View区域会发生什么?
了解了什么情况下会触发ACTION_CANCEL
,那么针对问题:滑出子View区域会触发ACTION_CANCEL
吗?这个问题就很明确了:不会。
实践是检验真理的唯一标准,代码撸起来:
public class MyButton extends androidx.appcompat.widget.AppCompatButton { @Override public boolean onTouchEvent(MotionEvent event) { switch (event.getAction()) { case MotionEvent.ACTION_DOWN: LogUtil.d("ACTION_DOWN"); break; case MotionEvent.ACTION_MOVE: LogUtil.d("ACTION_MOVE"); break; case MotionEvent.ACTION_UP: LogUtil.d("ACTION_UP"); break; case MotionEvent.ACTION_CANCEL: LogUtil.d("ACTION_CANCEL"); break; } return super.onTouchEvent(event); } }
一波操作以后日志如下:
(MyButton.java:32) -->ACTION_DOWN
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:36) -->ACTION_MOVE
(MyButton.java:39) -->ACTION_UP
滑出view后依然可以收到ACTION_MOVE
和ACTION_UP
事件。
为什么有人会认为滑出view后会收到ACTION_CANCEL
呢?
我想是因为滑出view后,view的onClick()
不会触发了,所以有人就以为是触发了ACTION_CANCEL
。
那么为什么滑出view后不会触发onClick
呢?再来看看View的源码:
在view的onTouchEvent()
中:
case MotionEvent.ACTION_MOVE: // Be lenient about moving outside of buttons // 判断是否超出view的边界 if (!pointInView(x, y, mTouchSlop)) { // Outside button if ((mPrivateFlags & PRESSED) != 0) { // 这里改变状态为 not PRESSED // Need to switch from pressed to not pressed mPrivateFlags &= ~PRESSED; } } break; case MotionEvent.ACTION_UP: boolean prepressed = (mPrivateFlags & PFLAG_PREPRESSED) != 0; // 可以看到当move出view范围后,这里走不进去了 if ((mPrivateFlags & PFLAG_PRESSED) != 0 || prepressed) { ... performClick(); ... } mIgnoreNextUpEvent = false; break;
1,在ACTION_MOVE
中会判断事件的位置是否超出view的边界,如果超出边界则将mPrivateFlags
置为not PRESSED
状态。
2,在ACTION_UP
中判断只有当mPrivateFlags
包含PRESSED
状态时才会执行performClick()
等。
因此滑出view后不会执行onClick()
。
结论:
- 滑出view范围后,如果父view没有拦截事件,则会继续受到
ACTION_MOVE
和ACTION_UP
等事件。 - 一旦滑出view范围,view会被移除
PRESSED
标记,这个是不可逆的,然后在ACTION_UP
中不会执行performClick()
等逻辑。
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